I am having the same problem on my Note8. The issue I believe, is the contact that is continually muted is marked as mute on verion messages online. I have not found a way to unmute a contact through the web browser. If this can be done I think it will solve the issue on my phone. Please advise, the contact is my wife, and she hates being ignored because she is muted!
after talking with verizon for over an hour, they couldn't solve it. I did find that the mute turned on instantly after sending a txt from my phone. So I knew it was something wrong with the app on my phone. I uninstalled Message +, then reinstalled and new it does not mute anymore! Fixed!